The objectives of this study were to examine the effects of management and environmental effects on milk yield (factors such as herd, year of calving, parity and age at calving), total and 305-dayand to examine the magnitude of the variability amongst cows. Data were obtained from three state farms. Data set consisted of records from 1004 Holstein-Friesian and 6690Brown Swiss cows. The average total milk yield, days in milk and 305 day milk yield were 5760 kg, 316 days and 5420 kg, respectively.  Repeatability of total milk yield, 305 day milk yield and most probable producing ability of each cow were calculated. Repeatability of total and 305 day milk yield was estimated to be 0.31. The results of this study suggest positive trends for lactation milk yield and the significant repeatability shows that it is possible to rank cows, on a within-herd basis, on the basis of their Most Probable Producing Ability, and hence use this as a phenotypic selection and culling criteria for herd management.